Interestingly enough, the “Responsive Design” units in the Front End Career Path on Codecademy were recently updated with some new info, so that’s definitely a good place to start, if you want to stay on-platform!
Currently, I’m auditing the courses Fundamentals of Graphic Design and Visual Elements of User Interface Design from CalArts on Coursera. (Debating whether I should pay to enroll officially for the assignment feedback and additional content.) I’ve also been reviewing some of my old notes from when I studied illustration in university–a lot of the design principles stay the same. In addition, I’ve been loving the book The Design of Everyday Things by Don Norman.
There’s a lot of good info out there, so I hope you can find some things that click for you!